Historical Articles

December, 1952 issue of Plating

Metal Recovery by lon Exchange

C. F. PAULSON
The Permutit Company, New York, N. Y.

ONE OF THE FIELDS in which
ion exchange holds the most promise is the recovery of metals. In a vast number
and variety of manufacturing processes, dilute solutions are produced containing
metals whose recovery would be most attractive.

We commonly think of metals
as cations when in solution, and it was logical for the first attempts to be
made using cation exchangers. However, some of the most important and valuable
metals are also frequently present as anions. Table I illustrates some of the
forms in which industrially important metals are present in solution. This list
is by no means exhaustive, but the eases given are typical, and will be expanded
upon to show the different methods of attack.

Metal Recovery from Acid
Solution
The examples of acid plant wastes containing small quantities of metals are
too numerous to mention. However, a large and typical one is zinc in the waste
from rayon plants. Zinc sulfate in an acid solution containing large quantities
of sulfuric acid and sodium sulfate is used to coagulate the cellulose strands
to form rayon, and when this solution is rinsed from the rayon, a large volume
of dilute solution is produced. Mindler has reported1 on tests using a sulfonated
crosslinked polystyrene resin. Many of the earlier types of cation exchangers
had been investigated unsuccessfully for this purpose. They would not remove
metals in the presence of hydrogen ions. The capacities at various acid concentrations
when treating solutions containing 500 ppm zinc are shown in Table II. It can
be seen that the capacity falls as the acid concentration rises. These data
show the capacity to a point where 50 ppm of zinc appears in the effluent. At
higher acid concentrations than 1 per cent, the slippage of zinc rose and the
capacity fell.

Using customary regeneration
techniques it would be possible to recover solutions containing several per
cent of zinc. However, if a much stronger solution can be recycled into the
manufacturing process, special regeneration techniques may be resorted to. The
counter current principle in a modified form is used to extract the zinc from
the cation exchanger at the most useful concentration. Several regenerant segments
are passed through the cation exchanger in order of decreasing zinc concentration
and increasing acid concentration. To achieve highest capacity, the final segment
must be a strong sulfuric acid solution, and to achieve high regenerant effluent
zinc concentration, many segments must be used.

Metal Recovery from Anionic
Complex by Cation Exchange
A novel application of cation exchange to metal recovery has been reported1.
In tin plating from alkaline baths, the rinse water contains recoverable amounts
of tin as sodium stannate. A sulfonated cross-linked polystyrene resin in the
hydrogen cycle was employed to treat such a rinse solution. Hydrogen ions were
exchanged for the sodium, causing reduction in pH and precipitation of metastannic
acid. The tin-bearing solution was sent through the bed upflow at a rate sufficient
to fluidize the bed. This prevented clogging of the voids in the bed by the
precipitate. Formation of an insoluble end product drove the reaction to completion,
and the very satisfactory capacity of 1400 meq/l was obtained. Sludge filtration
in a conical vessel was employed on the effluent solution to concentrate the
sludge. The overflow from this unit was water at a pH of 3.0 containing 30 ppm
of sodium and no trace of tin. This water could be neutralized and used for
further rinsing, being of better quality than most plant waters, and since the
entire process was conducted at elevated temperatures, the heat in the rinse
water was conserved.

The precipitate drawn from
the bottom of the sludge unit could be dissolved in strong hot caustic to yield
a solution satisfactory as makeup to a tin plating bath.

Recovery of Cationic
Complex by Cation Exchange
In the cuprammonium rayon industry vast quantities of copper-bearing water are
produced from rinsing the filaments. Most of this water is quite alkaline and
contains Cu probably as the tetramine, NH4, Na, S04, and
small quantities of other ions. Recovery of the copper in Germany by cation
exchange has been reported2 and a similar process is in operation
in this country. Since the quantity of copper is quite low, and the amount of
suspended material may be appreciable, the flow rate is high and the use of
a high capacity synthetic resin is not indicated. A sulfonated coal is used,
and at the high pH, its carboxylic acid and phenolic groups come into play giving
a capacity in the neighborhood of 750 meq/l of copper. The copper is taken up
eventually as the diamine, and since this is held very strongly by the exchanger,
the other cations, present in considerable excess compared to the copper, pass
through. The copper is then regenerated with 4 per cent sulfuric acid. The acid
solution containing copper sulfate may be readily recycled into the process,
but if the need arose, concentration along the same lines as described for zinc
could be performed.

Recovery of Chromium
by Cation Exchange
The processes mentioned above for copper and zinc can be applied very well to
the recovery of cationic chromium. However, relatively little of the chromium
which is produced by metal treating processes is initially present as the chromic
cation. Chromium is generally used as chromic acid in such processes as electroplating,
etching, anodizing, copper stripping, brightening after electroplating other
metals, etc. In these processes, the acid rapidly becomes contaminated with
metallic cations and must be discarded. Costa has described3 a prolonged
series of tests on the removal of interfering cations from chromic acid so that
the baths may be reused. This process makes use of one of the outstanding characteristics
of the sulfonated polystyrene cation exchange resins, their resistance to oxidation.
There have been published4, 5, 6 a number of papers on treating brass
and copper mill wastes by ion exchange but they almost all found that if hexavalent
chromium were present, both the cation and anion exchangers were oxidized and
lost their effectiveness. However, the polystyrene cation exchangers are resistant
to chromic acid up to 15 per cent and have an excellent capacity for metallic
cations in any solution of lower concentration.

In practice the method is
best applied to anodizing baths, where approximately one pound of aluminum is
dissolved for each pound that goes into the oxide coating on the metal, and
hard chrome plating baths where the prolonged contact of the strong acid solution
with the plated surface causes some of the cationic metal to go into solution.
By withdrawing each day a portion of this bath and treating it by cation exchange,
the bath may be permanently maintained at the point of maximum coating efficiency.
Former practice was to allow the metal content to build up to a point where
the coating was adversely affected and then dump a portion of the bath and refill
with fresh CrO3. This created a very serious waste disposal problem
and was expensive.

TABLE
II.CAPACITY OF PERMUTIT Q FOR ZINC

%
H2SO4

Total
Capacity, meq/l

Capacity
meq/l to 10% breakthrough

Zinc
Leakage, ppm

0.05

1430

2

0.1

1320

3

0.5

1040

20

1.0

830

15

1.5

643

100

3.0

308

155

5.0

115

254

Chromium Recovery by
Anion Exchange
An extension of the process described above has been developed recently. After
a chromic acid treatment, it is customary to rinse excess acid from the metal.
There are many ways of operating rinsing facilities with one or more flowing
rinses occasionally preceded by a still rinse. But in most cases a solution
must eventually be disposed of which contains from 10 to 100 ppm of chromate
anion. A new anion exchanger, Permutit S, is resistant to attack by such solutions
and will adsorb the chromate anion. The rinse water may then be recirculated.
The Permutit S is then regenerated with sodium hydroxide and the regenerant
effluent sent back to the anodizing tank through a cation exchange resin which
forms the free acid. In plant scale operations this process has been shown to
be profitable. Fig. 1 shows this equipment. The process conserves the chromic
acid, rinse water, and heat in the rinse water and eliminates a serious waste
disposal problem. It costs less than the purchase of an equivalent quantity
of chromic acid. In anodizing operations, it has also been shown to yield a
more corrosion resistant coating.

Fig. 1--Chromate
Recovery Equipment

Removal of Metals
from Acids by Anion Exchange
An interesting type of recovery has been reported by Kraus and Moore7. They
find that many metals customarily considered to be present in solution as cations
are present as anions in acid solution, actually probably as coordination compounds
with the acid. Such compounds as Fe(PO4)4-,FeCl4-,
and AlF6--- are typical. Even in acid as strong as 9 N
hydrochloric, these anions may readily be held by highly basic anion exchangers.
Kraus and Moore have applied their efforts chiefly to separations of hafnium,
columbium, tantalum, protoactinium, and zirconium, but have demonstrated the
ready manner in which metals which form coordination compounds can be separated
from those which do not. The process has been applied successfully in several
locations for the removal of traces of iron from muriatic acid.

Precious Metal Recovery
by Anion Exchange
Recovery of gold and silver from ore and electroplating wastes can be accomplished
by anion exchange. In these solutions the precious metal is generally present
in alkaline solution as the cyanide complex.: Conditions here are somewhat different
than in previous situations discussed as the very dilute form in which the precious
metals are present results in low capacity. Also, the complex precious metal
anions are only regenerated or the anion exchanger with difficulty. Efforts
are being directed toward development of special anion exchangers for precious
metals and effective special regeneration procedures have been developed.

Anion exchange has also
been attempted to recover gold from ore. Hussey has reported a series of investigations
along this line. Customary extraction processes depend upon the percolation
of a cyanide solution through the finely ground ore. However, an increasing
number of ore bodies being worked contain the gold mixed with clays and other
minerals which form slimes and prevent percolation. An anion exchanger may be
mixed with the ore during leaching. The anion exchanger holds the metal cyanide
as it is dissolved allowing complete leaching of the ore. The granular exchanger
may then be filtered from the slime and regenerated with caustic. Special ion
exchange equipment has been developed for this purpose.

TABLE
III, ANALYSIS OF HYPOTHETICAL. PLATING CYCLE WASTE EFFLUENT

Rinse

Copper

Nickel

Chromium

Flow gph

2000

2000

2000

Analysis-ppm Cu

8

Ni
22 CrO3

10

Na

12

S04 30 Cr+++

1

CN

14

Cl
3

Weight per day--lb
as metal

3.2

8.78

1.36

Value in solution--$/day

$3.18

$5.70

$0.92

Recovery of Electroplating
Wastes
To show how some of these techniques may be combined to solve a difficult disposal
problem, we will consider a plant doing chromium plating on top of nickel and
copper. Fig 2 shows the flow plan of such a plant and how ion exchange might
be incorporated. For simplification only one rinse is shown after each step;
except chromium plating, but more rinses might be necessary. The material to
be plated is cleaned, plated with copper from a cyanide bath, plated with nickel
from a sulfate bath, and plated with chromium from a chromic acid bath, with
rinses between each of the, steps. Various recovery methods including ion exchange
have been proposed for such a system, but they almost all were based upon treatment
of the mixed wastes and yielded some, if not all of the metal values in forms
which were difficult to recycle to the system. A typical system might involve
reduction of chromate, neutralization of the chromic cation and precipitation
with lime, and oxidation of the cyanide with chlorine.

Fig. 2--Plating
Recovery Flow Sheet

As analyses of individual
wastes from such plants are rather hard to locate, and generally vary from time
to time to such a degree as to be unreliable, the analyses in Table III have
been assumed.

Demineralized water is used
for making up all the baths and the copper rinse. The copper rinse, containing
the copper as a complex anion, is pumped from the overflow of the rinse tank
through a bed of a highly basic anion exchanger such as Permutit A which will
hold the cyanide, and copper complex. The effluent of this unit will contain
hydroxide equivalent to the anions removed and the run will be halted when cyanide
breaks through. Part of this water will be recy-cled and the remainder will
go to the nickel rinse bath. The highly basic anion exchanger will be regenerated
with caustic soda and the regenerant effluent which will be highly alkaline
will be returned to the plating bath. Regeneration rinse waters and any other
waters which might contain cyanides can be given a chlorine treatment to insure
freedom from toxicity and used to neutralize the acid rinse waters.

Fig--3. Closed System
Ion Exchange Installation

Rinse water from the nickel
rinse will go to the bed of sulfonated cross-linked polystyrene resin such as
Permutit Q where the nickel will be removed. At the low pH, little if any of
the sodium will be held. The effluent from this unit can be recycled to another
bath where traces of sodium sulfate would not be harmful. Upon regeneration
with sulfuric acid, this cation exchanger will yield a solution containing 2
per cent Ni and 2 per cent free H2SO4 which can be returned
to the plating bath. The excess acidity may have to be lowered by dissolving
nickel carbonate in the regenerant effluent before addition to the bath.

The chromate recovery system
will be a completely closed system. Fig. 3 shows such an ion exchange system.
The concentration of chromate in the still rinse will be allowed to rise to
about 10 per cent. At frequent intervals this still rinse will be passed through
a cation exchange unit to remove metallic cations, and occasionally part of
the treated acid will be returned to the plating tank. It may be necessary to
use an evaporator to concentrate this solution although experience in many locations
has indicated that a 10 per cent solution may be returned directly. While the
dragout from the plating rinse contains some metallic cations, the dragout from
the still rinse will be almost pure chromic acid. This running rinse will pass
through a highly basic styrene base anion exchanger and then be recycled. When
this unit is exhausted, it will be regenerated with caustic soda and the regenerant
effluent sent through the cation exchange unit for removal of sodium and then
to the still rinse tank. Throughout the cycle of the two units working on chromate
bearing liquors, it is possible to operate in such a manner that all the liquor
containing chromates will go to either the plating bath, still rinse or running
rinse and none will go to waste. The regenerant from the cation exchange unit
will be sent to waste as it will contain a mixture of metals which would not
be worth separating.

Let us examine the savings
that such a system would involve. It would be possible to oxidize the cyanides
with 7 parts of chlorine per part of cyanide, reduce the chromates with copperas
or sulfur dioxide and mix the three rinses and neutralize to precipitate the
hydroxides. These hydroxides could be removed by settling or filtration. A conventional
disposal house for these solutions would cost not less than $25,000. The sludge
from such a unit would contain approximately 5 per cent solids which could be
lagooned. It is possible that after settling, the precipitate could be sold
for reworking.

Neutralization would require
122 pounds per day of lime, reduction of chromate would require 216 pounds of
copperas, and oxidation of cyanide, 39 pounds of chlorine.

To treat the cyanide waste
would require 3 cu. ft. per day of anion exchanger. Actually the flow rate would
necessitate the use of a larger unit. A satisfactory unit would be 30 inches
in diameter and hold 14 cu. ft. of highly basic anion exchanger. Each day the
regenerant needed would be 4 pounds per cu. ft. each of caustic soda and sodium
cyanide.

The nickel recovery unit
would also have to be sized according to flow rate as approximately 4 cu. ft.
of styrene base cation exchanger per day would be needed. This unit also would
be 30 inches in diameter and would require 40 pounds per day of 66° Be H2SO4.

The chromate recovery anion
exchanger would be similar to the copper cyanide unit in size. It would require
5 pounds of caustic soda per day as regenerant. The chromate recovery cation
exchanger is difficult to estimate accurately as too many factors are involved
but it is assumed that operation would be similar to what was found in a chromic
acid anodizing plant where the sizes of anion and cation exchangers are similar.
Twenty pounds of sulfuric acid are used as regenerant for this unit. One extra
regeneration would be required to take care of anion regenerant so this unit
 would operate 2 cycles per seven days.

As on any day only one ion
exchange unit would require regeneration and this would require little attention,
no cost for labor has been included.

In estimating the value
of the recovered materials, the market prices of the chemical in solution is
used. Thus the value of the copper may seem rather high. Since chromium plating
baths are more concentrated than most others, dragout is more serious. Probably
if ion exchange were not used for this recovery, some other means of minimizing
this loss would be utilized, so the value of chromate recovered is somewhat
high. A cost balance indicates a saving of $3.82 per day over no waste
treatment or of $28.32 over a conventional system. This saving would pay for
the equipment in 300 working days. Also, since most of the water is reclaimed
and reused, the water cost will be reduced from $14.40 to $4.80 per day.

Recirculating Rinse Water
In many locations plating tank and piping layouts are such that individual rinse
solutions cannot be isolated for recovery of the valuable constituents. Here
also ion exchange can be profitably employed in the waste disposal scheme. Rather
than conventional waste holding tanks and chemical feeders, it is possible to
demineralize the entire waste stream, at an equipment cost comparable to the
tanks and feeders, to produce a high quality water for all plant processes.
The ion exchangers in the demineralization step may be regenerated when exhausted
to yield only a small volume of concentrated waste which may readily be rendered
non-toxic by conventional means. Such a process offers major economies in labor,
and of course the demineralized water results in freedom from staining and water
spots of the finished work, freedom from sludges in the baths and dull, pitted,
blistered, or brittle deposits. Frequently the cost of treatment for such recycled
water is less than the cost for treating the plant raw water to make it
suitable for plating operations.

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